RNA interference with a double-stranded short interfering RNA (siRNA) is highly promising not only as a research tool, but also as a therapeutic strategy, with its potent capacity of gene silencing (Non-patent Documents 1 to 4). However, applying siRNAs in vivo remains limited to topical delivery (Non-patent Documents 5 to 8). This limitation is attributable to the low stability of siRNA due to enzymatic decomposition in vivo and/or the low permeability of the cell membrane. While development of a delivery vehicle will possibly definitely enable the introduction of siRNA into target tissue by systemic administration, only a few reports are available on delivery systems which take advantage of the efficacy of siRNA on renal diseases.
A lipid-conjugated siRNA (Non-patent Document 9) or a liposome-encapsuled siRNA (Non-patent Document 10) has been shown to be accumulated in the liver and silence the target gene; however, the renal distribution of the siRNA is thought to be nothing more than watching the degradation process in tubular cells or the process of excretion into tubular lumen. Characterized by the secretion of a wide variety of pathogenic factors by resident cells in response to hemodynamic or immunological derangements, the glomerulus is a reasonable target for molecular therapy. However, an siRNA alone (a small molecule several nanometers long) is quickly excreted in the urine, whereas the above-described lipid-conjugated siRNA or liposome-encapsuled siRNA is difficult to deliver to mesangial cells and the like which are connective tissue in glomeruli, because of its size (several hundred nanometers) and characteristics. Against this background with the low availability of effective drugs for renal diseases, there is a demand for the development of a delivery system suitable for glomerulus-targeted gene silencing with siRNA.
The present inventors previously reported that a polyion complex with a block copolymer is useful as a delivery system for charged proteins and DNAs (Patent Document 1). The present inventors also investigated structures of block copolymers and methods of preparing a polyion complex that are particularly suitable for the delivery of double-stranded oligonucleic acids, and reported a block copolymer carrying a double-stranded oligonucleic acid in the form of a polymeric micelle (Patent Document 2). It was also reported that a complex of a carrier having as a side chain a hydrophilic group bound to a polycationic compound in the form of a comb (graft copolymer) and an RNA improves the stability and retentivity of the RNA in the blood (Patent Document 3). The nucleic acid delivery systems described in Patent Documents 1 and 2 are expected to ensure stable delivery to various tissues in vivo, and are polymeric micelles having a size distribution of several tens to several hundreds of nanometers. The complex described in Patent Document 3 is shown to potently suppress gene expression in the liver, but there is no description of delivery to kidney regions.